1) Field of the Invention
The present invention relates generally to depth control systems for agricultural implements, and, more specifically, to an improved series cylinder depth control circuit for wide tillage equipment and the like.
2) Related Art
Series cylinders are used extensively for controlling depth across the width of large agricultural implements such as field cultivators and chisel plows. Since the series cylinders extend and retract in unison, the implement can be raised and lowered in a relatively level fashion, and the working depth of the implement including hinged outer wing sections can be maintained generally uniform. However, several problems with presently available series circuits limit the accuracy of the working depth along the width of the implement.
Series cylinder circuits include cylinders with grooves to rephase the cylinders as they are cycled. The cylinders on the wings downstream of the main frame cylinder often fail to settle back past the rephasing grooves. The main frame cylinder is first to settle since it supports the heaviest load, and, as the main frame settles, the pressures downstream increase, often to a level such that the wing cylinders can support the wings and prevent the desired settling similar to that of the main frame cylinder. The settling problem is aggravated by the increased stand-by pressures on newer tractors.
A second problem is caused by hose expansion. When the machine is lowered to a preselected point determined by a mechanically actuated hydraulic check valve, system pressure in the series circuit increases to the stand-by pressure and causes all the holes to expand. If this stand-by pressure bleeds off through the tractor selective control valve (SCV) or if the implement is raised off the single point stop, the hoses all relax and hydraulic fluid is displaced from the main frame cylinder toward the downstream cylinders. In turn, each downstream cylinder moves from the desired working position, with the movement being cumulative such that each cylinder moves more than the previous cylinder. As a result, depth inaccuracies increase along the width of the implement. These inaccuracies can be as much as one inch or more, which is unacceptable in many field-working applications.
One attempt to reduce depth variations has been to insert a rebound valve, such as the commercially available Prince Model RB-4S-650, in the circuit between the input and output lines leading from the SCV to the series cylinders. The valve limits pressurization of the circuit to a level below full tractor pressure so that depth inaccuracies caused by hose expansion and relaxation is reduced. In addition to being fairly complicated and relatively expansive, rebound valves fail to solve the settling problem.